Surgical microscopes are used in operating rooms during surgeries on the organism of a human or an animal, for example, to produce magnified stereoscopic images of the patient's surgical area for the human eye. The surgical microscope is used primarily in neurosurgery and eye surgery; i.e., in particular where surgeries are to be performed on very fine structures of an organism, such as on blood vessels, in the area of the spine, eye, or on the brain.
Wireless footswitch devices for controlling medical technical equipment, and especially surgical equipment, which actually constitute actuators, have important advantages over cabled footswitches. On the one hand, cables are dangerous trip hazards which are to be avoided, especially in an operating room. Moreover, conventional cables are subject to increased wear because they are heavily stressed by pieces of equipment being rolled or pushed over them. As a result, the cables must be regularly replaced by suitably trained service personnel, which requires considerable maintenance effort. On the other hand, cables having a particularly wear-resistant sheath are costly and make the device more expensive. Furthermore, the cables become dirty very easily and are difficult to clean, which is why they often do not meet the high hygienic requirements of medical applications. Moreover, cables hinder the positioning of the footswitches. Therefore, free positioning of the footswitch is possible only to a limited extent and requires extra work, such as rerouting of the cable.
Therefore, footswitches with wireless signal transmission have been described which use, for example, infrared or radio technology for transmitting signals. However, due to the limited transmission capacity and range, infrared technology is inferior to radio technology.
European Patent Publication EP-A-2 033 591, for example, describes a wireless footswitch which uses Bluetooth radio technology for signal transmission. U.S. Pat. No. 7,428,439, in turn, describes a wireless footswitch in which uses infrared radiation for information transmission. In that approach, a power source supplies electrical power to a capacitor bank, which in turn supplies electrical power to a signal generation unit.
A drawback of wireless footswitches is that they must be equipped with an autonomous power supply, which is typically provided in the form of a battery supply. The batteries used for this purpose are either replaceable, non-rechargeable batteries or rechargeable secondary batteries (storage batteries), hereinafter generally referred to as “batteries”. However, in any case, the energy reserve is limited, and the batteries have to be periodically replaced or recharged.
In conventional wireless footswitches, the energy reserve of the batteries lasts from a few hours to a maximum of a few days. In order to prevent failure of the controller during use (e.g., during surgery), the operating personnel must constantly monitor the state of charge of the batteries. This situation is unsatisfactory, especially for applications in the surgical field.
The relatively short life of the batteries and the relatively high power consumption of conventional wireless footswitches are due to various reasons. For example, when using Bluetooth technology, a permanent radio link is established between the footswitch and the control unit within a particular frequency band. Bluetooth uses the frequency hopping method, in which the frequency band is divided into various discrete frequency channels, which are changed several times per second. The aforementioned characteristics of the Bluetooth technology have their advantages, but in this particular case they result in relatively high power consumption of the components. Consequently, frequent battery replacement or recharging is required.
In contrast to Bluetooth technology, ZigBee is a different, open standard for short-range wireless communication. ZigBee is a protocol stack which, in accordance with the so-called OSI model, is based on the PHY and MAC sublayers specified in the IEEE 802.15.4 standard. The protocol stack represents the conceptual architecture of network protocols in data transmission. The OSI layer model (Open Systems Interconnection Reference Model) is a specific layer model, which was developed by the International Organization for Standardization (ISO) as a basis for the design of communication protocols, such as ZigBee. The aforementioned IEEE 802.15.4 standard describes a transmission protocol and defines the two lowermost layers of the OSI model, which are referred to as physical layer (PHY) and media access control layer (MAC). The ZigBee standard defines the higher protocol layers, which provide the application interface.
Like other wireless standards (e.g., Bluetooth), ZigBee was developed for short-range wireless communication. These standards enable wireless connection of devices over short distances of, for example, from 1 to 50 meters. Advantageously, frequency transmission is implemented using the frequencies in the Industrial, Scientific and Medical (ISM) band. The ISM band was defined by the International Telecommunication Union, Radiocommunication Sector (ITU-R) and has been documented extensively in the prior art. Within the ISM band, specific frequency ranges are assigned to specific applications. For short-range wireless communication applications, for example, the frequency range between 2.402 GHz and 2.480 GHz is intended for medical-technical and industrial applications. This also applies to the ZigBee wireless standard. However, the assignment of such frequency ranges is purely administrative and not necessarily based on technical considerations. Accordingly, other frequency ranges, especially of the ISM band, could also be considered for short-range wireless communication in particular cases.